Salesforce Error 'User's page layout and language used by Executive Opportunity Alerts' When Trying to Deactivate a User

This particular error message is displayed because the user you are trying to deactivate is setup in the Big Deal Alerts for an Organization.  Before deactivating the User, the Administrator must replace this user in the Big Deal Alert settings.  

In Salesforce Classic:
1. Go to Setup | App Setup | Customize | Opportunities | Big Deal Alert
2. Click Edit.
3. Replace the user in the "User" lookup field under Sender section.


In Lightning Experience:
1. Go to Setup.
2. On the Quick Find Box, search "Big Deal Alert".
3. Replace the user in the "User" lookup field under Sender section.

Once the steps above are done, the User should be able to be deactivated.